This invention relates to body implantable medical devices, and more particularly to implantable electrodes for sensing electrical impulses in body tissue or for delivering electrical stimulation pulses to an organ, for example for pacing the heart or arresting tachycardia or cardioversion.
Cardiac pacing leads are well known and widely employed for carrying pulse stimulation signals to the heart from a battery operated pacemaker or other pulse generating means, as well as for monitoring electrical activity of the heart from a location outside of the body. More recently, electrodes have been used to stimulate the heart in an effort to terminate tachycardia or other arrhythmias. In all of these applications, it is highly desirable to minimize the electrical impedance at the interface between the electrode and body tissue.
A direct approach to reducing interface impedance is to increase the electrode surface area, which is subject to practical limits for maximum electrode size. Increasing the number of reactive sites in a electrode improves its ability to convert an electronic current to an ionic current. As used in this application, the term "impedance" relates to the conversion of electronic Current to ionic current.
One particularly effective means of increasing reactive surface area is to form a highly porous electrode body, for example as disclosed in U.S. Pat. No. 4,011,861 (Enger), and in U.S. Pat. No. 4,156,429 (Amundson). The Amundson Patent discloses a porous electrode formed by a bundle of fibers, preferably of platinum but alternatively of Elgiloy, titanium, or a platinum iridium alloy. The fibers are compressed into a bundle, then heated to a sufficient temperature and for a sufficient time to sinter the fibers. The fibers or filaments may be bundled within a metallic screen or grid, and preferably form between about three percent and thirty percent of the electrode volume, with the balance of the volume open. This macro porosity enhances ingrowth of tissue to stabilize the electrode, and the increased surface area to volume ratio lowers interface impedance, improving both sensing and pacing performance.
Other approaches to increasing electrode efficiency concern reducing fibrosis, i.e. formation of a capsule of inactive tissue that surrounds and isolates the electrode from active tissue. The resultant increase in distance from the electrode to viable tissue increases the voltage required to generate the same transmembrane potential. In U.S. Pat. No. 4,281,668 (Richter et al), a vitreous carbon or pyrolytic carbon electrode is superficially activated, e.g. by oxidation, for micro porosity. The electrode then is coated with a body compatible, ion conducting and hydrophobic plastic. This approach is said to substantially prevent thrombus formation.
U.S. Pat. No. 4,603,704 (Mund et al) discloses an electrode including a hemispherical head of platinum or titanium. A porous layer is coated over the head, either by vapor deposition or by magnetron sputtering. The porous layer consists of a carbide, a nitride or a carbonitride of at least one of the following metals: titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum or tungsten.
U.S. Pat. No. 4,542,752 (DeHaan et al) features an implantable lead with a core of platinum, titanium or similar metal, covered with a porous sintered titanium alloy, which in turn is covered with a porous carbon lattice. The porous carbon surface is said to promote tissue ingrowth and provide low polarization impedance.
In U.S. Pat. No. 4,407,302 (Hirshorn et al), the external surface of a cardiac pacer electrode tip is provided with a concavity and roughened over its exterior, for example by abrading with a jet of glass beads, to increase the micro surface area of the electrode tip and reduce the sensing impedance of the tip. At the same time, the concave area in an otherwise convex surface of the electrode tip is said to significantly and advantageously increase the pacing impedance. The underlying theory of this approach, with respect to pacing impedance, is that higher resistance reduces the current flow for a given voltage, and consequently reduces the energy involved in pacing.
An example of a porous electrode tip is found in U.S. Pat. No. 4,577,642 (Stokes), in which the electrode is formed by sintering spheres or other particles of metal resulting in formation of molecular sieves which control the elution rate of a drug housed in the lead distal end. This approach, however, requires a balancing between a relatively large reactive surface area and pore size of the structure. Sintering small spheres enhances surface area but reduces porosity. Conversely, sintering of larger spheres results in a more porous structure with lower surface area. In any event, maximum theoretical porosity is under fifty percent, and the pores or passages typically are tortious and convoluted.
Despite the varying degrees of success of the above approaches, polarization losses and after potentials remain significant problems to electrode efficiency. Depending on applied potential and pulse duration, activities at the electrode interface range from reorganization of ions to electrolysis. As current densities increase, these reactions change the ionic concentration at the interface, requiring migration of ions from increasingly greater distances. The energy required to reorient and move the ions is the measure of the polarization loss of the electrode, and represents wasted energy for a loss in efficiency. The source of the after potential is the concentration gradient or residual charge at the end of a pulse.
Therefore, it is an object of the present invention to provide a body implantable electrode with substantially reduced polarization loss, and reduced capacitive coupling at the electrode/tissue interface, thereby reducing signal distortion.
Another object is to provide an electrode construction with reduced after potential, thus reducing the refractory period and reducing sensing delays following stimulating pulses.
Another object is to provide an electrode having large, non-tortious pores open to the electrode exterior in combination with a microscopic texturing of exterior and interior surfaces of the electrode.
Yet another object is to provide an intravascular pacing lead having a reduced chronic threshold, improved pulse sensing capability and shorter recovery time for sensing after stimulation pulses.